Make no mistake, business and legal analysts say, escort advertising is a lucrative business generating tens of millions of dollars a year.

And it has law enforcement scrambling to keep up, as evidenced in the recent discovery of four women whose bodies were found in trunks of cars on Detroit's east side over the holidays. Police have learned three of the victims had ties to escort ads -- a revelation that has homicide investigators combing the Internet for clues, looking at one salacious ad after another.

They're not hard to find.

And research shows the titillating ads rake in big bucks.

In the last 12 months, five websites alone sold roughly $36.1 million in ads for escorts and body rubs, according to Advanced Interactive Media Group, a consultant to the classified advertising industry based in Altamonte Springs, Fla. The sites were Backpage.com, which sold the most -- $24.3 million in ads -- Eros.com, CityVibe.com, MyRedBook.com and AdultSearch.com.

Detroit police have said three of the victims were connected to escort ads on Backpage.com. On Thursday, the company said it has evidence 22 other websites also were involved.

According to the AIM Group, Backpage.com became the leading site for escort advertising when Craigslist pulled the plug on adult services ads in September 2010 following pressure from lawmakers and clergy leaders.

The AIM Group spent the last 12 months tracking escort advertising on 24 websites in 23 cities, including Boston, Chicago, Denver and Washington; Detroit was not on the list. Its research showed Craigslist was on track to generate $44.6 million from escort ads in 2010 before eliminating the service.

Escorts, however, quickly found other venues.

According to the AIM Group, the number of escort ads on seven websites jumped by nearly 27%, from 189,300 ads in 2010 to 240,000 this year.

"These are not secret sites. Police seem to know even about these smaller sites," said Mark Whittaker, a senior consultant for the AIM Group.

To determine their revenue, Whittaker said researchers used simple math: They counted the ads they found on sites and then multiplied them by the published advertising rates, which vary site to site, ranging from $5 a post to $12 a week. In some cases, the ads are free, but the subscribers pay to view them.

Backpage.com, which has come under pressure from lawmakers and clergy over the past year to eliminate its adult-services ad section, denied AIM's claims that it is the leading online seller of escort ads. Representatives for the other four websites could not be reached.

Backpage.com would not disclose how much money it makes from escort ads, nor would it say whether AIM's figure of $24.3 million was too high or too low. The company did, however, stress it has the legal right to sell the ads.

"We just disagree with the notion that the Internet is the problem here. If you want to prevent street crime, do you close the streets? The Internet isn't the problem -- the criminals are," said Backpage attorney Steve Suskin.

Suskin also defended the company's policing tactics, saying Backpage closely monitors the ads for criminal behavior and works with law enforcement.

"Backpage does not believe its website poses any threat to any individual, in the same way that cell phone providers, the postal service and the interstate highway system do not," Suskin said.

Critics aren't buying it.

Over the last year, politicians and clergy have called on Backpage.com to stop running escort ads. A team of 46 attorneys general -- including Michigan's -- are demanding Backpage explain how it handles escort postings and prove it's monitoring the site for illegal activity.

In an August letter to the company, the attorneys general said Backpage.com is a hub for prostitution and child trafficking, and has failed to police itself, citing several examples.

• In May, a Dorchester, Mass., man was charged with forcing a 15-year-old girl into a motel to have sex with men for $100 to $150 an hour. To find customers, the man posted a photo of the girl on Backpage.com.

• In Washington, prosecutors are handling a case in which teen girls say they were coerced, threatened and extorted by two adults who marketed them on Backpage.com.

• In the last three years, criminal charges have been filed in more than 50 cases against suspects accused of trafficking or trying to traffic minors on Backpage.com.

Most recently, the operators of Escorts.com pleaded guilty last month in federal court in Pennsylvania to money laundering and agreed to forfeit $4.9 million, pay a $1.5-million fine, and give up the escorts.com domain name. Prosecutors claimed that prostitutes and escort services used the site to connect to clients.

"Just take it down," said Anny Donewald, a former prostitute and stripper who now runs Eve's Angels, a statewide support group for women in the adult entertainment business.

Donewald said she believes web operators put women in danger by letting them post ads that go unmonitored. Such ads, she said, give predators easy access.

"Why even give them a venue?" Donewald said. "Take it down. ... Is the $25 per ad worth lives?"

Online escort ads also have drawn the anger of Michigan Attorney General Bill Schuette.

"The level of activity on such websites for prostitution and the sexual exploitation of children is very concerning," said Schuette spokesman John Sellek, who hopes political pressure will lead to "real action by the companies to put an end to this as soon as possible."

Legal experts say it may be hard to do.

Web operators have a First Amendment right to run ads in which adults can arrange to meet each other. In addition, they're shielded from liability for what others say or post on their sites under the Communications Decency Act.

That's the law one federal judge in Missouri cited in August when he dismissed a teenager's lawsuit against Backpage.com over nude photos a pimp posted of the child on the site. The judge held that while the minor endured "horrific victimization" at the hands of her pimp, Backpage.com was protected by federal law.

Tampa attorney Paul DeCailly, who represents escort services, said he believes that websites -- like newspapers -- shouldn't be sued over ads. He said escort ads are like personal ads: People are looking to meet people. What happens afterward isn't the fault of the middleman who ran the ad, he said.

DeCailly also noted that websites like Backpage.com provide a business service for many companies looking for new clients, not just escorts.

In fact, he advertises his law practice on the site twice -- in the legal section, and the one for escorts.